In manufacturing a semiconductor device, there is a film-forming step wherein a film is formed on a substrate to be processed, such as a semiconductor wafer, by a CVD or the like. As one of processing apparatuses for carrying out the film-forming step, there is known a vertical type of thermal processing apparatus, which is a batch type of processing apparatus that can thermally process a large number of wafers at a time.
The processing apparatus consists of: a boat (holder) that holds wafers in a tier-like manner; a reaction tube (processing container) that contains the boat and that conducts a predetermined thermal process such as a CVD to the wafers in a process-gas (reaction-gas) atmosphere under a predetermined temperature and pressure; a gas-introducing part that introduces a process gas into the reaction tube; a gas-discharging part that discharges a gas from the reaction tube to create a predetermined vacuum pressure therein; and a heater (heating part) that heats the reaction tube. In the processing apparatus structured like the above, the process gas is introduced from one end, such as a lower end, of the reaction tube, rises up in the reaction tube, passes through a substrate-holding area thereof, and is discharged from the other end, such as an upper end, of the reaction tube.
In the processing apparatus, depending on process conditions (temperature, gas flow rate, pressure, processing time), absorption amount of process-gas molecules is greater at an area nearer to the introduction side of the process gas. Thus, it is difficult to obtain sufficient film-forming uniformity among the substrates in the same batch.
On the other hand, an ALD (Atomic Layer Deposition) process, which is one of film-forming processes, is a process in which a film is formed on a substrate by causing a plurality of kinds of gases to flow independently within a short time and sequentially into the reaction tube. In the ALD process, in order to improve the productivity, it is necessary to efficiently purge the gas within a short time after the absorption of the process gas. However, in the above processing apparatus, it is difficult to efficiently purge the gas staying between the substrates. That is, it is not sufficient to achieve the improvement of the productivity.
In Japanese Patent laid-Open Publication No. 2000-208425, there is described a processing apparatus (semiconductor-device manufacturing apparatus) in which a process gas is horizontally supplied from gas nozzles disposed at one lateral side of wafers held on a boat and is discharged from absorption holes disposed oppositely to the gas nozzles. In the processing apparatus, circular barrier plates are provided inside the reaction tube and outer circular parts corresponding to the circular barrier plates are provided at wafer-holding shelf portions of the boat, in such a manner that a layer of the process gas ejected from one gas nozzle doesn't mix with the other layers of the process gas effected from the upper or lower gas nozzles substantially.
However, since this processing apparatus is provided with the circular barrier plates inside the reaction tube and with the outer circular parts at the wafer-holding shelf portions of the boat, the structure is complicated and the size of the apparatus including the reaction tube is unavoidably enlarged.
In addition, in each wafer-holding shelf portion of the boat, pins are provided to protrude therefrom for supporting a wafer at a higher level in order to cause a part of the process gas flowing on or above the wafer to flow under the wafer, so that a film formed on an under surface of the wafer can have a sufficient thickness. Furthermore, in each wafer-holding shelf portion of the boat, a plurality of large openings is formed to expose the reverse (under) side of the wafer. Thus, it is possible that the upper layer of the process gas and the lower layer of the process gas substantially mix with each other through the openings, so that it is difficult to obtain sufficient film-forming uniformity among the substrates. In addition, it is also difficult to efficiently purge the gas staying between the substrates.